However, the GFP chromophore may enjoy its own local mobility that may not be coupled to the protein's rotation [1]}. In that case, it is informative to check Perrin-Weber plot of \(1/r\)  as a function of \(T/\eta \) . When the chromophore's mobility is strongly coupled to the viscosity, \(1/r\)  is positively correlated and varies linearly with \(T/\eta \) . When the probe enjoys local mobility, \(1/r\)  can become a non-linear function of \(T/\eta \)  [2]} and an increasing slope at large values of \(\eta \)  (smaller values of \(T/\eta \) ) would mean larger local mobility [2]}, [1]}, [5]}. In Fig. REF , we show \(1/r\)  as a function of \(T/\eta \)  for four different temperatures. The viscosity \(\eta (p,T)\)  were calculated as described above. We find that \(1/r\)  is non-linear function of \(T/\eta \)  for all the temperatures studied here, Moreover, for \(T=20^{\circ }\) C, \(1/r\)  displays a positive correlation for \(p\le 80\)  MPa and negative correlation for larger pressures, resulting in a nose-shaped curve due to the viscosity anomaly of water [6]}, [7]}, [8]}. For \(T\ge 30^{\circ }\) C, \(1/r\)  decreases monotonically with \(T/\eta \) . It is interesting to note that for low temperature (\(T<30^{\circ }\) C), \(1/r\)  is positively correlated with \(T/\eta \)  up to the pressure where the viscosity anomaly of the water disappears. Viscosity of diffusion anomaly in water at low temperatures has been attributed to breaking of hydrogen bonds with pressure and subsequent saturation of disordered hydrogen bond network at higher pressures [9]}, [10]}. It is likely that H-bonding environment around the chromophore is affected at high pressure resulting in the changes in radiative and non-radiative decay channels.
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